I have a class that runs a function inside the init/constructor that loops over an array of...
struct Answer {
let correct: Bool
let text: String
}
...and searches for an object that has the correct boolean variable set to true. The loop and search is done via a function named getCorrectId() that returns a index to where the Answer with the correct: true is placed.
The function is implemented like this:
private func getCorrectId() -> Int? {
for i in 0..<self.answers.count {
if self.answers[i].correct {
return i
}
}
return nil
}
As you can see, the function returns an optional integer. In 99.9% of the cases it should not fail, and if it fails it is because I've made an logical programming error by myself somewhere in the code.
SolutionHandler class, including init() and getCorrectId() function
struct Answer {
let correct: Bool
let text: String
}
class SolutionHandler {
var correctId: Int?
var answers = [Answer]()
init(correctSolution: String, incorrectSolutions: [String]) {
// Add the correct solution to the answers array
self.answers.append(Answer(correct: true, text: correctSolution))
// Add the incorrect solutions to the answers array
for incorrectSolution in incorrectSolutions {
self.answers.append(Answer(correct: false, text: incorrectSolution))
}
// Shuffle the answers to make them random in array
self.answers.shuffle()
<-- IMPORTANT PART
// Find the correct id (with forced unwrap)
self.correctId = getCorrectId()!
-->
}
private func getCorrectId() -> Int? {
for i in 0..<self.answers.count {
if self.answers[i].correct {
return i
}
}
return nil
}
}
So, to my main question... how should I unwrap this optional? Is it okay to force unwrap it or should I make the class SolutionHandler be set to nil if getCorrectId() function fails and handle it whenever I create the class? Or should I maybe throw an custom error and catch it whenever I create this class?
Thanks in advance! :)
You already know which one is correct; it's always the first one, as guaranteed by the way you constructed the answers array.
I think a better solution to the problem would be to not mix together all answers into a single array. Something like this:
class SolutionHandler { // FIXME: this is probably a bad name for this, and it should probably be a struct.
let correctAnswer: Answer
let incorrectAnswers: [Answer]
var answers: [Answer] { incorrectAnswers + [correctAnswer] }
init(correctSolution: String, incorrectSolutions: [String]) {
self.correctAnswer = Answer(correct: true, text: correctSolution)
self.incorrectAnswers = incorrectSolutions.map { Answer(correct: false, text: $0) }
}
}
I would do something like this:
if result = getCorrectId() {
self.correctId = result
}
Only assign the result of getCorrectID() if it is not nil.
Related
I'm trying to do something I think should be pretty simple, but I'm running into trouble with Swift's type inference. I really don't understand why it's falling down here.
I have a type Cocktail, which has other properties, but the only one important here is the name:
struct Cocktail {
// ... other stuff
let name: String
}
Then I have two protocols:
protocol ScrollIndexable {
var scrollIndexTitle: String { get }
}
protocol ScrollIndexProviding {
var scrollIndices: [any ScrollIndexable] { get }
}
along with a simple conformance on String to ScrollIndexable:
extension String: ScrollIndexable {
var scrollIndexTitle: String { self }
}
I want to make it so that I can use an array of Cocktails as a ScrollIndexProviding:
extension Array: ScrollIndexProviding where Element == Cocktail {
var scrollIndices: [any ScrollIndexable] {
let firstCharacters = reduce(into: Set<String>()) { partialResult, cocktail in
guard let firstCharacter = cocktail.name.first else {
return
}
partialResult.insert(String(firstCharacter))
}
// The return line here has two errors:
// Cannot convert return expression of type 'Array<Cocktail>' to return type '[any ScrollIndexable]'
// No exact matches in call to initializer
return Array(firstCharacters)
}
}
This extension fails to build, with two errors:
Cannot convert return expression of type 'Array' to return type '[any ScrollIndexable]'
No exact matches in call to initializer
The second error seems like noise to me, since Set conforms to Sequence, so I should be able to use that init method.
The first error is confusing to me since the firstCharacters array is of type Set<String>, so the error message just doesn't seem to make any sense. Is there something I'm misunderstanding about the any keyword here? What's going on?
The issue is that you're inside an extension of Array where the Element is Cocktail, so when you try to create an array without specifying the element type the compiler will assume you mean for the element type to be Cocktail.
extension Array where Element: Cocktail {
func someMethod() {
// This array is of type `Array<Cocktail>` since the compiler
// assumes the array's element type should be the same as
// Self's element type, which (from the extension) is `Cocktail`.
let array = Array()
}
}
So, to fix this, just explicitly tell the compiler that the array's element type is String, as in:
extension Array: ScrollIndexProviding where Element == Cocktail {
var scrollIndices: [any ScrollIndexable] {
let firstCharacters = reduce(into: Set<String>()) { partialResult, cocktail in
guard let firstCharacter = cocktail.name.first else {
return
}
partialResult.insert(String(firstCharacter))
}
return Array<String>(firstCharacters)
// ^^^^^^^^ add this
}
}
I have create a class called notification Item and parsing the data from model class RTVNotification
import Foundation
import RTVModel
public class NotificationItem: NSObject {
public var id: String
public var title: String
public var comment: String
public var publishStartDateString: String
init(id: String,
title: String,
comment: String,
publishStartDateString: String) {
self.id = id
self.title = title
self.comment = comment
self.publishStartDateString = publishStartDateString
super.init()
}
}
extension NotificationItem {
static func instantiate(with notification: RTVNotification) -> NotificationItem? {
return NotificationItem(
id: notification.id,
title: notification.title,
comment: notification.comment,
publishStartDateString: notification.publishStartDateString)
}
}
ViewModel
public class SettingsViewModel: ViewModel {
var item = [NotificationItem]()
public var fetchedNotifications: Driver<NotificationItem> = .empty()
public var apiErrorEvents: Driver<RTVAPIError> = .empty()
public var notificationCount: Driver<Int> = .empty()
public func bindNotificationEvents(with trigger: Driver<Void>) {
let webService: Driver<RTVInformationListWebService> = trigger
.map { RTVInformationListParameters() }
.webService()
let result = webService.request()
apiErrorEvents = Driver.merge(apiErrorEvents, result.error())
notificationCount = result.success().map {$0.informationList.maxCount }
fetchedNotifications =
result.success()
.map {$0.informationList.notifications}
-----> .map {NotificationItem.instantiate(with: $0)}
}
}
Getting an Error saying that Cannot convert value of type '[RTVNotification]' to expected argument type 'RTVNotification'
What can i do to solve this.
The purpose of the map() function is to iterate over the elements of an input array and apply a transform function to each of those elements. The transformed elements are added to a new output array that is returned by map(). It's important to understand that the length of the output array is the same length as the input array.
For example:
let inputArray = ["red", "white", "blue"]
let outputArray = inputArray.map { $0.count } // outputArray is [3, 5, 4]
In your code, you are calling:
result.success().map { $0.informationList.notifications }
I don't know RxSwift at all, so I'm going to go into wild speculation here.
First, I don't know exactly what result.success() returns, but the fact you can call map() on it implies result.success() returns an array (which is weird, but ok we'll go with it).
Second, we know the array returned by result.success() contains elements that have an informationList property, and the informationList property has a property called notifications. My guess is that notifications, being plural, means the notifications property type is an array, probably [RTVNotification].
So this code:
result.success().map { $0.informationList.notifications }
Transforms the success() array into a new array. Based on my assumption that notifications is of type [RTVNotification], and further assuming the success() array contains only one element, I would expect the result of
result.success().map { $0.informationList.notifications }
To be an array of type [[RTVNotification]], i.e. an array with one element, where that element is an array of RTVNotifications.
You then feed that [[RTVNotification]] array into another map() function:
.map { NotificationItem.instantiate(with: $0) }
Recall from the start of this answer that map() iterates over the elements of arrays. Since the input array to this map is [[RTVNotification]], its elements will be of type [RTVNotification]. That's what the $0 is in your call - [RTVNotification]. But the instantiate(with:) function takes an RTVNotification, not an array of RTVNotification, thus you get the error:
Cannot convert value of type '[RTVNotification]' to expected argument type 'RTVNotification'
So what can you do to fix it?
I would probably do something like this (you'll have to tailor it to your use case):
guard let successResponse = webService.request().success().first else {
print("no success response received")
return nil // basically report up an error here if not successful
}
// get the list of notifications, this will be type [RTVNotification]
let notifications = successResponse.informationList.notifications
// Now you can create an array of `[NotificationItem]` like you want:
let notificationItems = notifications.map { NotificationItem.instantiate(with: $0) }
// do something with notificationItems...
The caveat to the above is if you need to iterate over each element in the success() array, then you could do that like this:
let successResponses = webService.result().success()
// successNotifications is of type [[RTVNotification]]
let successNotifications = successResponses.map { $0.informationList.notifications }
// successItems will be of type [[NotificationItem]]
let successItems = successNotifications.map { notifications in
notifications.map { NotificationItem.instantiate(with: $0) }
}
In other words, in this last case, you get back an array that contains arrays of NotificationItem.
Your problem is here:
fetchedNotifications: Driver<NotificationItem> should be fetchedNotifications: Driver<[NotificationItem]> and the line .map {NotificationItem.instantiate(with: $0)} needs another map You are dealing with an Observable<Array<RTVNotification>>. You have a container type within a container type, so you need a map within a map:
.map { $0.map { NotificationItem.instantiate(with: $0) } }
When your types don't match, you need to change the types.
Other issues with your code...
Drivers, Observables, Subjects and Relays should never be defined with var, they should always be lets. Objects that subscribe to your properties before the bind is called will connect to the .empty() observables and never get any values. This is functional reactive programming, after all.
Your NotificationItem type should either be a struct or all it's properties should be `let's.
Be sure to read and understand #par's answer to this question. He wrote a really good explanation and it would be a shame to waste that knowledge transfer.
I have this question except for Swift. How do I use a Type variable in a generic?
I tried this:
func intType() -> Int.Type {
return Int.self
}
func test() {
var t = self.intType()
var arr = Array<t>() // Error: "'t' is not a type". Uh... yeah, it is.
}
This didn't work either:
var arr = Array<t.Type>() // Error: "'t' is not a type"
var arr = Array<t.self>() // Swift doesn't seem to even understand this syntax at all.
Is there a way to do this? I get the feeling that Swift just doesn't support it and is giving me somewhat ambiguous error messages.
Edit: Here's a more complex example where the problem can't be circumvented using a generic function header. Of course it doesn't make sense, but I have a sensible use for this kind of functionality somewhere in my code and would rather post a clean example instead of my actual code:
func someTypes() -> [Any.Type] {
var ret = [Any.Type]()
for (var i = 0; i<rand()%10; i++) {
if (rand()%2 == 0){ ret.append(Int.self) }
else {ret.append(String.self) }
}
return ret
}
func test() {
var ts = self.someTypes()
for t in ts {
var arr = Array<t>()
}
}
Swift's static typing means the type of a variable must be known at compile time.
In the context of a generic function func foo<T>() { ... }, T looks like a variable, but its type is actually known at compile time based on where the function is called from. The behavior of Array<T>() depends on T, but this information is known at compile time.
When using protocols, Swift employs dynamic dispatch, so you can write Array<MyProtocol>(), and the array simply stores references to things which implement MyProtocol — so when you get something out of the array, you have access to all functions/variables/typealiases required by MyProtocol.
But if t is actually a variable of kind Any.Type, Array<t>() is meaningless since its type is actually not known at compile time. (Since Array is a generic struct, the compiler needs know which type to use as the generic parameter, but this is not possible.)
I would recommend watching some videos from WWDC this year:
Protocol-Oriented Programming in Swift
Building Better Apps with Value Types in Swift
I found this slide particularly helpful for understanding protocols and dynamic dispatch:
There is a way and it's called generics. You could do something like that.
class func foo() {
test(Int.self)
}
class func test<T>(t: T.Type) {
var arr = Array<T>()
}
You will need to hint the compiler at the type you want to specialize the function with, one way or another. Another way is with return param (discarded in that case):
class func foo() {
let _:Int = test()
}
class func test<T>() -> T {
var arr = Array<T>()
}
And using generics on a class (or struct) you don't need the extra param:
class Whatever<T> {
var array = [T]() // another way to init the array.
}
let we = Whatever<Int>()
jtbandes' answer - that you can't use your current approach because Swift is statically typed - is correct.
However, if you're willing to create a whitelist of allowable types in your array, for example in an enum, you can dynamically initialize different types at runtime.
First, create an enum of allowable types:
enum Types {
case Int
case String
}
Create an Example class. Implement your someTypes() function to use these enum values. (You could easily transform a JSON array of strings into an array of this enum.)
class Example {
func someTypes() -> [Types] {
var ret = [Types]()
for _ in 1...rand()%10 {
if (rand()%2 == 0){ ret.append(.Int) }
else {ret.append(.String) }
}
return ret
}
Now implement your test function, using switch to scope arr for each allowable type:
func test() {
let types = self.someTypes()
for type in types {
switch type {
case .Int:
var arr = [Int]()
arr += [4]
case .String:
var arr = [String]()
arr += ["hi"]
}
}
}
}
As you may know, you could alternatively declare arr as [Any] to mix types (the "heterogenous" case in jtbandes' answer):
var arr = [Any]()
for type in types {
switch type {
case .Int:
arr += [4]
case .String:
arr += ["hi"]
}
}
print(arr)
I would break it down with the things you already learned from the first answer. I took the liberty to refactor some code. Here it is:
func someTypes<T>(t: T.Type) -> [Any.Type] {
var ret = [Any.Type]()
for _ in 0..<rand()%10 {
if (rand()%2 == 0){ ret.append(T.self) }
else {
ret.append(String.self)
}
}
return ret
}
func makeArray<T>(t: T) -> [T] {
return [T]()
}
func test() {
let ts = someTypes(Int.self)
for t in ts {
print(t)
}
}
This is somewhat working but I believe the way of doing this is very unorthodox. Could you use reflection (mirroring) instead?
Its possible so long as you can provide "a hint" to the compiler about the type of... T. So in the example below one must use : String?.
func cast<T>(_ value: Any) -> T? {
return value as? T
}
let inputValue: Any = "this is a test"
let casted: String? = cast(inputValue)
print(casted) // Optional("this is a test")
print(type(of: casted)) // Optional<String>
Why Swift doesn't just allow us to let casted = cast<String>(inputValue) I'll never know.
One annoying scenerio is when your func has no return value. Then its not always straightford to provide the necessary "hint". Lets look at this example...
func asyncCast<T>(_ value: Any, completion: (T?) -> Void) {
completion(value as? T)
}
The following client code DOES NOT COMPILE. It gives a "Generic parameter 'T' could not be inferred" error.
let inputValue: Any = "this is a test"
asyncCast(inputValue) { casted in
print(casted)
print(type(of: casted))
}
But you can solve this by providing a "hint" to compiler as follows:
asyncCast(inputValue) { (casted: String?) in
print(casted) // Optional("this is a test")
print(type(of: casted)) // Optional<String>
}
I want to write a function named remove in Swift which will accept an array or a string and remove the string from a dictionary if it is a string else it will remove all the strings from the dictionary which are present in the array. Also, this function that I wrote is disabling the styles in the editor of Xcode.
func remove(key: AnyObject){
if key is Array{
for (index, value) in enumerate(key){
if -1 < self._getDataStoreKeyIndex(value){
self._removeProperty(value)
} else{
self._removeItem(value)
}
}
}else{
if -1 < self._getDataStoreKeyIndex(key){
self._removeProperty(key)
}else{
self._removeItem(key)
}
}
}
The other functions in the code are correct because if I comment this function my project is building successfully. What is wrong in my code?
I am getting a segmentation fault.
<unknown>:0: error: unable to execute command: Segmentation fault: 11
The heart of the problem is attempting to convert to "Array" which is a generic class. Instead, you need to convert to a specific instantiation of the generic, in your case "Array" or "[String]"
You also have a problem in that you can't enumerate AnyObject, combined, you need something like:
func remove(key: AnyObject) {
if let array = key as? Array<String> {
for (index, value) in enumerate(array) {
}
}
else {
}
}
Although I'm not sure why you're using enumerate to get at the indices which you don't use for anything, faster and more legible to just iterate the array directly:
func remove(key: AnyObject) {
if let array = key as? Array<String> {
for value in array {
}
}
else {
}
}
One further thought... these two operations really aren't similar and don't really share much code. Why not define to different functions that differ in signature:
func remove(key:String) {
}
func remove(array:[String]) {
for string in array {
remove[string]
}
}
Which eliminates the possibility that somebody calls "remove(5)"
I'm trying to copy the all function from python to swift, starting off with checking for any nill items in a list, but I'm having a tough time checking optional items. For some reason I can send a optional string (string for example) and even though it says it's nil it still passes thru an if statement, which it doesn't outside of the function. Any advice about how to deal with this or another way of doing it? Thanks!
func `all`<T>(array: [T]) -> Bool {
for item in array {
if item as Any? {
println(item) // Says Nil >.<
}
var test: T? = item
if test {
println("Broken") // Prints broken :(
}
}
return true
}
var t: String?
all([t])
It's unclear to me exactly what you're trying to test, but maybe this will help.
The parameter to the function should be an Array of optionals [T?]
It may also be beneficial to directly compare elements to nil. The comparison could be abstracted to a closure much like the filter function uses.
func all<T>(array: [T?]) -> Bool {
for element in array {
if element==nil {
return false
}
}
return true
}
I know this is old, but it still being searched for. Just spent couple of hours looking for a solution and I think I have finally found one.
if (item as AnyObject) is NSNull {
//value is nil
}